Superalloys are metal alloys usually based on nickel or cobalt having high tensile strength and resistance to fatigue at high temperatures. These alloys, therefore, have potential use in the manufacture of turbopump blades for the Space Shuttle main engines, as well as blades for aircraft, marine and aerospace gas turbines.
Directional solidification of alloy castings has been achieved heretofore by the progressive advance of a solidification front between the solid and the liquid phase which permits the growth of a reinforcement phase in the form of dendrites.
Many improvements on this process have been attempted in the prior art. U.S. Pat. No. 4,057,097 discloses a method of unidirectional solidification comprising casting a melt of the alloy into a mold, followed by progressive cooling with a temperature gradient along the length of the casting until a metastable equilibrium is reached, the entire body of the melt being in a super-cooled state as a homogeneous liquid. This super-cooled liquid is then instantly solidified by disturbance of the metastable equilibrium.
U.S. Pat. No. 4,540,038 discloses a two-step solidification process for turbine blades. The airfoil section of the turbine blades is solidified at a slow rate so as to effect directional solidification, while the root section of the turbine blade is solidified with magnetic stirring at a faster rate than applied heretofore so as to eliminate any inhomogeneous portion at the interface between the airfoil and root sections.
U.S. Pat. No. 3,669,180 discloses the production of fine-grained ingots of superalloy by solidification of a well stirred two-phase liquid/solid mixture in two separately controlled thermal zones. The upper zone is controlled to maintain the mixture liquid while the lower zone is being solidified, whereby the liquid fills any shrinkage due to the solidification below it.
Thus far, there has been no satisfactory process available for solidifying of superalloys and creating a fine microstructure which results in improved fatigue resistance.